FIELD OF THE INVENTION
The present invention relates to a scroll type fluid machine, namely volume type fluid machines used for a compressor, a vacuum pump, an expander or the like and, more particularly, a scroll type fluid machine suitable to provide a high performance and a high reliability in various uses.
The fundamental principle of the scroll type fluid machine is well known. As a shape of a spiral wrap of the scroll type fluid machine, an involute shape based on a circle having a constant diameter as shown in FIG. 6 has been commonly used since it is easy for machining. Japanese Patent Application Laid-Open No. SHO 57-73803 shows an example of such scroll type fluid machines.
Fundamental elements of the scroll type fluid machine of this type include a fixed scroll and an orbiting scroll, each of which has a same spiral shape consisting of an involute based on a same circle having a constant diameter, a suction port formed in the fixed scroll at the outside of the orbiting scroll, a discharge port formed in the fixed scroll at a central part thereof, a rotation preventing mechanism for preventing rotation of the orbiting scroll to cause an orbiting movement of the orbiting scroll relative to the fixed scroll and a driving mechanism for driving the orbiting scroll.
Japanese Patent Application Laid-Open No. SHO 60-252102 discloses a construction of the scroll type fluid machine of this kind in which a thickness of the spiral wrap continuously varies from its starting end to its terminating end.
In the conventional scroll type fluid machine in which a spiral body of each scroll is formed by an involute curve based on a circle having a constant radius, a freedom in determining the spiral shape of the wrap is limited when a radius of a base circle, an involute angle, a thickness of the wrap and a height of the wrap are determined, and a stroke volume (a volume at the time when a confinement effected by an outermost part of the wrap has been completed) and a built-in volume ratio (inner volume ratio) are determined thereby. Thus, there are problems as hereafter described.
In case of a compressor for a refrigerator which is operated under the conditions where a ratio (pressure ratio) between a suctioning pressure and a discharge pressure is high, the built-in volume ratio must be high and, in order to secure a high built-in volume ratio, a winding angle must be larger, thereby resulting in increasing an external size of the compressor. If the winding angle is increased while the external size and the height of the spiral body are held at predetermined values, the thickness of a plate of the spiral body is reduced and, consequently, a strength is lowered or the stroke volume is reduced.
In general, a pressure difference between operating chambers becomes higher toward a central part where a fluid is compressed and a pressure of the fluid is increased. In case of the conventional scroll type fluid machine as described above, the thickness of plate of the spiral body is uniform and, therefore, in order to compensate for the lowering of the strength it is necessary to uniformly decrease the height of the plate thickness of the spiral body or uniformly increase the plate thickness of the spiral body. Accordingly such problems occur that some part becomes unnecessarily thick or the size of radius becomes unnecessarily large.
In a scroll type fluid machine as shown in the Japanese Patent Application Laid-Open No. SHO 60-252102, the thickness of the spiral wrap varies from its starting end to its terminating end, but it has been found that no consideration is given to a phase or other conditions, so that the fixed scroll and the orbiting scroll have different curves. Therefore, it is necessary to effect machining of the orbiting scroll and the fixed scroll according to different machining programs. There is another problem in that a contact point between an outer line of the spiral wrap of the orbiting scroll and an inner line of the spiral wrap of the fixed scroll is located at a position out of a tangential line relative to the base circle and, therefore, a complete sealing point is not always obtained. There is a further problem in that a groove width of the spiral body varies, depending upon the winding angle and, therefore, it is necessary, at the time of machining the spiral body by means of an end mill, to separately effect machining of the inside surface and the outside surface of the spiral body and to effect a plurality of machining steps to form a bottom surface of the groove, depending upon a variation of the width of the groove. Otherwise, the spiral body cannot be precisely manufactured. Thus the number of machining steps is increased.